Regents Chemistry Exam Explanations January 2003

41 Which phase change represents a decrease in entropy?

(1) solid to liquid (3) liquid to gas
(2) gas to liquid (4) solid to gas

S	L	G
1------------>
2 <-----------
3 ------------->
4 ----------------------->

42 Given the equation:

2C₂H₂ (g) + 5O₂ (g) → 4 CO₂(g) + 2H₂O(g)

How many moles of oxygen are required to react completely with 1.0
mole of C₂ H₂ ?

(1) 2.5 (3) 5.0
(2) 2.0 (4) 10

1 mole	=	X
2C₂H₂	=	5 O₂

43 A student intended to make a salt solution with a concentration of 10.0
grams of solute per liter of solution. When the student’s solution was
analyzed, it was found to contain 8.90 grams of solute per liter of solution.
What was the percent error in the concentration of the solution?

(1) 1.10% (3) 11.0%
(2) 8.90% (4) 18.9%

10.0 - 8.90	x 100%
10.0

44 What is the molarity of a solution of NaOH if 2 liters of the solution contains 4 moles of NaOH?

(1) 0.5 M (3) 8 M
(2) 2 M (4) 80 M

4 moles	=
2 liters

Temperature must be in Kelvin

V	=	V
T	=	T

6 According to Reference Table J, which of these metals will react most
readily with 1.0 M HCl to produce H₂(g)?

(1) Ca (3) Mg
(2) K (4) Zn

The higher up on table J

47 The graph below represents the heating curve of a substance that starts as a solid below its freezing point.

What is the melting point of this substance?

(1) 30°C (3) 90°C
(2) 55°C (4) 120°C

The lower plateu is the melting/freezing phase change. Look at teh temp. there.

The upper plateu should be longer and it represents vaporization (requires more energy),

48 Given the unbalanced equation:

__Fe₂O₃ + __CO → __Fe + __CO₂

When the equation is correctly balanced using the smallest whole-number coefficients, what is the coefficient of CO?

(1) 1 (3) 3
(2) 2 (4) 4

1:3:2:3

alcohol -OH only

Since energy is a reactant it will consume the added energy and shift right, therefore increasing the [NO₂].